Field of Invention
The present invention relates to a flyback power converter with a programmable output and a control circuit and a control method thereof; particularly, it relates to such an output-programmable flyback power converter which maintains a same or relatively higher operation frequency and provides the same output current when its programmable output voltage switches to a relatively lower voltage, and a control circuit and a control method thereof.
Description of Related Art
A prior art output-programmable flyback power converter converts an input voltage to a programmable output voltage according to a setting signal, wherein the level of the output voltage is programmed according to different requirements of a load circuit. The programmable output voltage switches between at least two different levels according to the setting signal. In the prior art, when the output voltage is switched to a relatively lower level, compared to a relatively higher level, under the condition that the output current maintains the same, the output power will decrease whereby an operation frequency of an operation signal controlling a power switch of the flyback power converter decreases, and therefore, the equivalent output impedance decreases. That the equivalent output impedance decreases will cause the bandwidth of the overall system to increase, and decrease the phase margin of the system. In general, the phase margin is preferably higher than 45 degrees in a stable system. The phase margin of the aforementioned prior art output-programmable flyback power converter may become lower than 45 degrees or even a negative phase when it switches at the relatively lower output level, and the system is unstable.
In view of the above, the present invention proposes a flyback power converter with a programmable output which, as it provides a same output current, maintains a same or relatively higher operation frequency when a programmable output voltage switches to a relatively lower voltage, and a control circuit and a control method thereof.